Apparatus and method for recovering energy after carbon dioxide capture

ABSTRACT

Disclosed are an apparatus and a method for recovering energy after carbon dioxide capture. The apparatus includes an energy recovery unit at a discharge part of a carbon dioxide capturing apparatus through which captured carbon dioxide is discharged. The energy recovery unit reduces a discharge pressure of the carbon dioxide to a pressure level suitable for a fixation or conversion treatment, and simultaneously generates and recovers energy generated during the pressure reduction.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of Korean Patent Application No. 10-2012-0024182 filed Mar. 9, 2012, the entire contents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present invention relates to an apparatus and method for recovering energy after carbon dioxide capture. More particularly, it relates to an apparatus and method for recovering energy after carbon dioxide capture, which can recover energy from a discharge pressure of captured carbon dioxide when the captured carbon dioxide is treated by a method such as fixation or conversion.

(b) Background Art

Generally, methods for capturing carbon dioxide include an absorption method, an adsorption method, and a separation membrane method. Absorption methods can treat a large amount of exhaust gas compared to adsorption methods and separation membrane methods, and provides a high removal efficiency even in cases when the CO₂ concentration condition is about 7% to about 30%. Further, adsorption methods have a high economical efficiency and are easy to apply.

The carbon dioxide, once captured, can then be stored, or can be treated by fixation and conversion methods. Among these, the method of storing CO₂ in the ground or deep sea is easy even with a large amount of CO₂, in contrast to the other treatment methods. Such storage methods are currently available are have been commercialized. However, the cost for storing CO₂ in the ground or deep sea is high, and the stored CO₂ cannot be fundamentally removed, making additional profit-making difficult.

The methods of converting captured CO₂ into other chemical substances using CO₂ as a carbon source, and fixing CO₂ using plants and seaweeds are both being studied. If such methods reach a commercialization stage, CO₂ can be fundamentally removed, and useful products produced thereby can allow for additional profit-making. Accordingly, these methods are being evaluated as more economical and preferable technologies.

Among the absorption methods for capturing carbon dioxide, a chemical absorption method is currently being most widely developed. In a chemical absorption method, CO₂ is selectively separated from exhaust gas by a chemical reaction. With chemical absorption, the amount of absorption is not significantly affected by the CO₂ partial pressure. Accordingly, there is an advantage in that the CO₂ removal efficiency is high even when the CO₂ partial pressure is low. However, the chemical absorption method is limited because high energy consumption is required in a subsequent recovery process in which CO₂ is separated from an absorbent. For example, it is known that the energy cost for recovery accounts for about 60% or more of the total CO₂ recovery cost of a CO₂ capturing apparatus. In particular, it is known that the energy cost for separating CO₂ from absorbent in a recovery tower accounts for about 80% of the energy cost for CO₂ recovery, and the energy cost for maintaining process equipment such as a pump accounts for about 20% of the energy cost for CO₂ recovery.

Accordingly, an improved absorption technology is needed for capturing carbon dioxide wherein energy consumed in absorbent recovery is reduced, thereby reducing the cost for collecting carbon dioxide.

Hereinafter, a typical carbon dioxide capturing processing will be described in brief.

As shown in FIG. 2, an exhaust gas containing CO₂ is supplied to an absorption tower 10 that has a wide surface area for smooth gas-liquid contact and which is filled with filling substances.

In this case, a liquid absorbent is supplied from an absorbent storage tank 12 to an upper part of the absorption tower 10, and an exhaust gas is supplied to a lower part of the absorption tower 10. the exhaust gas contacts the liquid absorbent (absorption solution) at an atmospheric pressure in the upper end of the absorption tower 10, allowing CO₂ in the exhaust gas to be absorbed into the absorption solution, generally within a temperature range of 40° C. to 70° C.

The absorbent that absorbs CO₂ is discharged from the absorption tower 10 and is supplied to a recovery tower 14 where it undergoes a recovery process in which the absorbent is heated to a temperature of 100° C. to 160° C. Thereafter, the absorbent is discharged from the lower part of the recovery tower 14 (“used CO₂ absorbent”) and it is resupplied to the absorption tower 10 through an absorbent supplying line 22.

Absorbent that is resupplied to the absorption tower 10 is heated by passing through a heat exchanger 16. As shown, absorbent newly supplied to the recovery tower 14 from the absorbent storage tank 12 can be preheated by heat exchange with the heated absorbent that is resupplied from the lower part of the recovery tower 14. This combined heated absorbent is then supplied to the upper part of the recovery tower 14.

During the recovery process in which absorbent is heated to a temperature of 100° C. to 160° C. in the recovery tower 14, evaporated absorbent and CO₂ is discharged from the upper part of the recovery tower 14. Absorbent with CO₂ is discharged from the lower part of the recovery tower 14 and is heated to a temperature range of 100° C. to 160° C. by a heater 18, such as a boiler, to separate CO₂ from the absorbent.

CO₂ separated in the recovery tower 14 is discharged through a condenser to locations for storage, fixation, and conversion, and the evaporated absorbent is condensed in the condenser 20 then fed back to the recovery tower 14.

The CO₂ separated in the recovery tower 14 is a high concentration of gaseous CO₂ (90% to 100%), and is discharged from the recovery tower 14 at a pressure range of 1.9 atm to 6 atm to be finally treated by a storage, fixation, or conversion method.

In order to store captured CO₂ in the ground or deep sea, the pressure of a high concentration of CO₂ discharged from the upper end of the recovery tower 14 must be increased to a high pressure of about 70 atm to about 100 atm. For this pressure increase, additional energy is required.

On the other hand, when captured CO₂ is directly treated by fixation or conversion instead of storage, the captured CO₂ can be treated by a pressure of just 1.2 atm or less and, thus, a process of increasing the pressure of CO₂ with a compressor is unnecessary.

The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

SUMMARY OF THE DISCLOSURE

The present invention provides an apparatus and method for recovering energy after carbon dioxide capture, which reduces a discharge pressure of captured CO₂ to a pressure necessary for fixation or conversion. The apparatus and method further simultaneously generates energy (e.g. in a generator connected to a turbine), wherein the energy is generated by fixation or conversion of the captured CO₂ instead of storing the captured CO₂ in the ground or deep sea. This generated energy can be supplied to process operating units of the present apparatus and, thus, can be used by any of the operating units to capture CO₂.

In one aspect, the present invention provides an apparatus for recovering energy after carbon dioxide capture, including an energy recovery unit at a carbon dioxide discharge part of a carbon dioxide capturing apparatus, wherein the energy recovery unit reduces a discharge pressure of the carbon dioxide to a pressure level suitable for a fixation or conversion treatment. According to various embodiments, energy generated during the pressure reduction can be simultaneously recovered by the energy recovery unit.

In an exemplary embodiment, the energy recovery unit may be in connection with one or more process operating units of the carbon dioxide capturing apparatus to supply the recovered electrical energy to the desired process operating units.

In another exemplary embodiment, the energy recovery unit may include: a turbine disposed at an outlet of a condenser, wherein the outlet is a discharge part of the carbon dioxide capturing apparatus; and a generator connected to the turbine.

In another aspect, the present invention provides a method for recovering energy after carbon dioxide capture, including: capturing, by a carbon dioxide capturing apparatus, carbon dioxide from an exhaust gas; discharging, by the carbon dioxide capturing apparatus, the captured carbon dioxide; reducing a discharge pressure of the discharged carbon dioxide to a pressure level suitable for a fixation or conversion treatment; and recovering energy generated during the pressure reduction.

In an exemplary embodiment, the method may further include supplying the recovered energy to one or more desired process operating units of the carbon dioxide capturing apparatus to utilize the recovered energy.

In another exemplary embodiment, the energy recovery may include: rotating a turbine using the discharge pressure of the carbon dioxide captured by the carbon dioxide capturing apparatus; continually reducing a final discharge pressure of the carbon dioxide that has passed the turbine to the pressure level suitable for the fixation or conversion treatment; and delivering a rotary force of the turbine to a generator connected to the turbine to enable generation of energy by the generator.

In still another exemplary embodiment, when the discharge pressure of the carbon dioxide captured by the carbon dioxide capturing apparatus ranges from about 1.8 atm to about 6 atm, the final discharge pressure of the carbon dioxide that has passed the turbine may be reduced to a pressure of less than about 1.8 atm, less than about 1.6 atm, less than about 1.4 atm, or a pressure of about 1.2 atm which is suitable for the fixation or conversion treatment.

Other aspects and exemplary embodiments of the invention are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIG. 1 is a diagram illustrating an apparatus for recovering energy after carbon dioxide capture according to an embodiment of the present invention; and

FIG. 2 is a diagram illustrating a typical carbon dioxide capturing apparatus.

Reference numerals set forth in the Drawings includes reference to the following elements as further discussed below:

-   -   10: absorption tower     -   12: absorbent storage tank     -   14: recovery tower     -   16: heat exchanger     -   18: heater     -   20: condenser     -   22: absorbent supplying line     -   30: energy recovery unit     -   32: turbine     -   34: generator

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Hereinafter reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. While the invention will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention to those exemplary embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g., fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles.

The above and other features of the invention are discussed infra.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The present invention provides an apparatus and method that improves the economical efficiency of CO₂ capture by reducing the CO₂ absorption cost, particularly by reducing energy costs.

According to the present invention, a CO₂ capturing apparatus is designed to discharge captured CO₂ for conversion or fixation after reducing the discharge pressure of the CO₂ to a pressure level suitable for fixation or conversion treatment, wherein energy is recovered during the pressure reduction, and the recovered energy is supplied to one or more of process units of the CO₂ capturing apparatus.

According to an exemplary embodiment, as shown in FIG. 1, an energy recovery unit 30 may be disposed around a location where CO₂ captured by the CO₂ capturing apparatus is discharged. For example, the energy recovery unit 30 may be disposed at the side of an outlet of a condenser 20 connected to a recovery tower 14 of the CO2 capturing apparatus.

When CO₂ captured in the CO₂ capturing apparatus is discharged from the outlet of the condenser 20, the energy recovery unit 30 may be configured and arranged to reduce the discharge pressure of CO₂ to a pressure level suitable for fixation or conversion treatment, and to recover energy generated during the pressure reduction.

More specifically, the energy recovery unit 30 according to an embodiment of the present invention may include a turbine 32 disposed at an outlet of the condenser 20 that is a discharge part of the CO₂ capturing apparatus. A generator 34 can be provided in connection with the turbine 32, for example, by concentrically connecting the generator 34 to the turbine 32 or by other suitable arrangements.

The generator 34 of the energy recovery unit 30 may be connected to one or more process operating units (e.g., pump and blower disposed in each capture process, which are typically driven by electrical energy) of the CO₂ capturing apparatus so as to supply generated electrical energy to the respective process operating units.

Hereinafter, a method of recovering energy after CO₂ capture according to an embodiment of the present invention will be described as follows.

As shown in FIG. 1, exhaust gas containing CO₂ may be supplied into an absorption tower 10, and absorbent, typically liquid absorbent, may be supplied from an absorbent storage tank 12 to an upper part of the absorption tower 10.

The exhaust gas supplied into the absorption tower 10 may contact liquid absorbent (absorption solution), typically at an atmospheric pressure, in the absorption tower 10 (e.g. in the upper part of the absorption tower 10), and CO₂ within the exhaust gas may be absorbed by the absorbent.

The absorbent that absorbs CO₂ (“used CO₂ absorbent”) is discharged from the absorption tower 10, and is supplied to a recovery tower 14 where it may then undergo a recovery process. In particular, in the recovery process the absorbent is heated to a suitable temperature (such as a temperature of about 100° C. to about 160° C.) in the recovery tower 14.

The absorbent recovered in the recovery process is discharged from the lower part of the recover tower 14, and may then be resupplied to the absorption tower 10 via an absorbent supplying line 22 which connects the absorbent storage tank 12 and the absorption tower 10.

As shown, the resupplied absorbent passes through a heat exchanger 16, and thereafter combines with CO₂ absorbent newly supplied from the absorption tower 10. As such, the newly supplied CO₂ absorbent may be preheated by heat exchange with the heated resupplied absorbent, and the combined absorbent (newly supplied absorbent and resupplied absorbent) may then be supplied to the upper part of the recovery tower 14.

During the recovery process in which absorbent is heated to a suitable temperature, such as a temperature of about 100° C. to about 160° C., evaporated absorbent with CO₂ may be discharged from the upper part of the recovery tower 14. Further, liquid absorbent with CO₂ may be discharged from the lower part of the recovery tower 14, may pass through a heater 18 (e.g. a boiler or the like) where it is heated to a suitable temperature range, such as a temperature of about 100° C. to about 160° C., so as to separate CO₂ from the absorbent.

CO₂ separated in the recovery tower 14, i.e., CO₂ with evaporated absorbent, may be discharged to a condenser 20. From the condenser, condensed absorbent may be resupplied to the recovery tower 14, while separated CO₂ may be discharged to a location for fixation or conversion treatment.

When separated CO₂ is discharged from the condenser 20 to the location for the fixation or conversion treatment, the pressure of CO₂ may range from about 1.8 atm to about 6 atm. A suitable discharge pressure of CO₂ necessary for the fixation or conversion treatment may be less than this discharge pressure, and, for example, may be less than 1.8 atm, less than 1.6 atm, less than 1.4 atm, and in some embodiments, may be about 1.2 atm.

As shown in the embodiment of FIG. 1, separated CO₂ discharged from the condenser 20 at a pressure of about 1.8 atm to about 6 atm is passes through the turbine 32 of the energy recovery apparatus 30. As the separated CO₂ passes through the turbine, the turbine 32 may be rotated, and the rotary force of the turbine 32 may be delivered to the generator 34.

While the separated CO₂ discharged from the condenser 20 passes through the turbine 32, the pressure of CO₂ may be reduced to a suitable pressure level for the fixation or conversion treatment. In particular, according to an exemplary embodiment, separated CO₂ is discharged from the condenser 20 at a pressure of about 1.8 atm to about 6 atm, and passes through the turbine 32 where the pressure of the CO₂ is constantly or continuously reduced as needed to a suitable pressure level for fixation or conversion treatment.

For example, the final discharge pressure of CO₂ that has passed through the turbine 32 may be reduced to a pressure of about 1.2 atm, which is a suitable pressure for the subsequent fixation or conversion treatment.

As the CO₂ passes through the turbine and is reduced in pressure, the rotary force of the turbine 32 may be delivered to the generator 34, enabling the generation of energy by the generator 34. Electrical energy generated in the generator 34 may be supplied to and consumed in one or more of the process operating units (e.g., pump and blower disposed in each capture process and driven by electrical energy) of the CO₂ capturing apparatus.

As a result, the amount of energy that must be supplied (i.e. external energy) to operate the CO₂ capturing apparatus can be significantly reduced, and costs can be saved by utilizing electrical energy generated in the generator 34 of the energy recovery unit 30 as energy for powering one or more of the process operating units of the CO₂ capturing apparatus.

As a test example of the present invention, a test of energy recovery was performed using a process simulation program in which the amount of CO₂ capture (removal) was about 1000 ton/day. The CO₂ absorption process conditions are shown in Table 1 below.

TABLE 1 Gas-liquid flow ratio 125 150 175 200 225 Flow rate of exhaust gas, 2,125.6 2,125.6 2,125.6 2,125.6 2,125.6 m3/min CO₂ concentration, 20 20 20 20 20 mol % Flow rate of absorbent, 17.0 14.2 12.1 10.7 9.4 m3/min MEA concentration in 35 35 35 35 35 absorption tower, wt %

The consumed energy kW of a reboiler (e.g., heater 18 connected to the lower part of the recovery tower 14) for each gas-liquid flow ratio and the flow rate of CO₂ gas discharged from the condenser are shown in Table 2 below.

TABLE 2 Gas-Liquid Flow Ratio 125 150 175 200 225 Amount of 1,000 1,000 1,000 1,000 1,000 CO₂ Removal, ton/day Energy Used 697.2 577.2 489.8 425.0 376.0 in Absorbent Pump, kW Flow Rate of 77.95 79.50 80.88 81.97 83.50 CO₂ discharged from Condenser, m3/min Pressure of 4.41 4.32 4.25 4.20 4.12 CO₂ discharged from Condenser, atm

The simulation results of energy generated through the turbine for each gas-liquid flow ratio according to the above test conditions are shown in Table 3 below.

TABLE 3 Division Wet radical flow Dry radical flow Gas-Liquid Flow Ratio 125 150 175 200 225 125 150 175 200 225 Pressure of 4.41 4.32 4.25 4.20 4.12 4.41 4.32 4.25 4.20 4.12 CO₂ injected into turbine, ata Pressure of 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 1.20 CO₂ discharged from turbine, atm Turbine 0.75 0.75 0.75 0.75 0.75 0.85 0.85 0.85 0.85 0.85 efficiency Generated 177.3 176.1 175.1 174.2 173.0 223.9 222.4 221.0 220.0 218.5 energy, kW Reduction rate 25.4 30.5 35.7 41.0 46.0 32.1 38.5 45.1 51.8 58.1 of energy used in absorbent pump, %

As shown in Table 3, the pressure of CO₂ inputted into the turbine ranged from about 4.12 atm to about 4.41 atm regardless of a wet or dry flow, and the pressure of CO₂ discharged into a fixation or conversion treatment unit through the turbine is constantly reduced to about 1.20 atm. Also, as energy generated by the generator increased according to the turbine efficiency, energy used in the absorbent pump of the CO₂ capturing apparatus was reduced.

According to the embodiments of the present invention, when CO₂ captured by the CO₂ capturing apparatus are processed by fixation or conversion treatment instead of a method of storing CO₂ in the ground or deep sea, the discharge pressure of CO₂ captured by the CO₂ capturing apparatus can be reduced to a pressure necessary for the fixation or conversion treatment. An energy recovery unit can be provided to generate energy from the pressure reduction, particularly wherein a turbine is positioned through which captured CO₂ passes such that the rotary force of the turbine can be delivered to a generator to obtain an energy recovery effect in which electrical energy is produced.

Also, since electrical energy produced in the generator can be utilized as energy for driving various process operating units (e.g., pump and blower) of the CO₂ capturing apparatus, energy (i.e., externally supplied energy) necessary for operating the apparatus to capture CO₂ can be significantly saved.

The invention has been described in detail with reference to exemplary embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents. 

What is claimed is:
 1. An apparatus for recovering energy after carbon dioxide capture, comprising an energy recovery unit at a discharge part of a carbon dioxide capturing apparatus, the discharge part configured for discharging captured carbon dioxide, wherein the energy recovery unit reduces a discharge pressure of the carbon dioxide and simultaneously recovers energy generated during the pressure reduction.
 2. The apparatus of claim 1, wherein the discharge pressure is reduced to a pressure suitable for a fixation or conversion treatment of the captured carbon dioxide.
 3. The apparatus of claim 1, wherein the energy recovery unit is connected to one or more process operating units of the carbon dioxide capturing apparatus and is configured and arranged to supply the recovered energy to one or more of the process operating units.
 4. The apparatus of claim 1, wherein the apparatus further comprises a condenser at the discharge part, and the energy recovery unit comprises: a turbine disposed at an outlet of the condenser; and a generator connected to the turbine.
 5. A method for recovering energy after carbon dioxide capture, comprising: capturing, by a carbon dioxide capturing apparatus, carbon dioxide from an exhaust gas; discharging, by the carbon dioxide capturing apparatus, the captured carbon dioxide, the discharged carbon dioxide having a discharge pressure; reducing the discharge pressure of the discharged carbon dioxide to a pressure level suitable for a fixation or conversion treatment; and recovering energy generated during the pressure reduction.
 6. The method of claim 5, further comprising supplying the recovered energy to one or more process operating units of the carbon dioxide capturing apparatus.
 7. The method of claim 5, wherein the step of recovering energy comprises: flowing discharged carbon dioxide through a turbine, thereby rotating the turbine with the discharge pressure of the discharged carbon dioxide to create a rotary force; reducing the pressure of the carbon dioxide that has flowed through the turbine to a reduced pressure level suitable for the fixation or conversion treatment; and delivering the rotary force of the turbine to a generator connected to the turbine and allowing generation of energy in the generator.
 8. The method of claim 7, wherein when the discharge pressure of the carbon dioxide captured by the carbon dioxide capturing apparatus ranges from about 1.8 atm to about 6 atm, the reduced pressure of the carbon dioxide is less than about 1.8 atm.
 9. The method of claim 8, wherein the reduced pressure of the carbon dioxide is less than about 1.6 atm.
 10. The method of claim 8, wherein the reduced pressure of the carbon dioxide is less than about 1.4 atm.
 11. The method of claim 8, wherein the reduced pressure of the carbon dioxide is about 1.2 atm. 